Heating, refrigeration or air-conditioning systems that typically employ rotary screw compressors are commonly found in office buildings, hotels, shopping malls, food storage and processing facilities, chemical processing, a wide variety of manufacturing plants, etc. Keeping the operating cost low is a key job of building owners as well as manufacturing plant managers. In this regard, the ability to keep heating and cooling cost low is critical; ASHRAE (American Society of Heating, Refrigeration and Air-Conditioning Engineers) estimates that 50% of building energy consumption is due to heating or cooling. In addition, there are important positive environmental and social impacts if energy efficiency can be improved.
The rotary screw compressor is a type of positive displacement compression machine. The key parts of the conventional rotary screw compressor are composed of two meshing, parallel and helical-profiled rotors housed in a casing. The movement and design of the rotors allow gas to be drawn into, sealed off and compressed as the gas is transported from the suction port to the discharge port thereof. In the design of oil-filled rotary screw compression machines, a lubricating oil bridges the gap between the rotors therefore providing a hydraulic seal and transferring mechanical energy between the driving and driven rotors.
The basic requirements of rotary screw compressors are robustness and reliability as they are expected to have a life expectancy of years or even decades while operating continuously with little maintenance work. The robustness and energy efficiency of rotary screw compressors have been achieved and are continually pursued by mechanical engineers in precise fitting and tight clearances between the helical rotors, and between the rotors and the chamber for better sealing of the compression cavities; and in precise assembly with the help of adoptions of advanced material and ever progressing modern digital controllers with intelligent algorithm. However, lubricant can be a relatively cost effective way to achieve higher efficiency. An optimal outcome would be a lubricating oil that improves both the efficiency and reliability of a compressor.
Evaporator is a critical component in refrigeration or air-conditioning systems; it is in the evaporator that the actual cooling or heat transfer takes place. The evaporators are a type of heat exchangers that transfer heat between substances, which in the case of air-conditioners between refrigerant and air, to be cooled by the refrigerant. A common necessity of evaporators is that heat transfer surfaces, whether inside or outside the evaporators, need to be kept clean in order not to impede the heat transfer through conduction. For example, defrosting is commonly deployed for this purpose. The impact of a lubricating oil on heat transfer arises from the fact that lubricating oil is pumped out of the discharge port along with the discharged gas. Many refrigeration systems or machines typically have oil separators built in downstream from the discharge port, but these oil separators can't capture all lubricating oil molecules. It is inevitable that some lubricating oil would pass through the separator. The passing lubricating oil needs to be drawn back to the compressor; otherwise any accumulation of lubricating oil in areas outside the compressors may cause oil starvation inside the compressor and/or hinder system functionality such as heat transfer in the evaporator of the refrigeration or air-conditioning system. Lubricating oils which are designed to be compatible/miscible with the refrigerant, allowing it to circulate back to the compressor along with the refrigerant, and having little tendency of accumulation on evaporator surfaces, are highly desired.
There are contradicting requirements of lubricating oil which is capable of providing better hydraulic sealing while in possession of excellent compatibility/miscibility with refrigerants, for example, while high viscosity lubricating oil typically provides more effective hydraulic sealing and wear protection, it tends not to have adequate compatibility or miscibility with the refrigerant especially at low temperatures. A serious, adverse implication of this inadequate compatibility/miscibility is poor oil return and reduced heat transfer in the low temperature evaporator/heat exchanger area.
The challenges in the development of high viscosity, performance-improving lubricating oil is further complicated by the adoption of non-ozone depleting environmentally friendly hydrofluorocarbon (HFC) refrigerants. The conventional chororflorocarbon (CFC) or hydrochoroflorocarbon (HCFC) refrigerants, owning to the presence of chlorine atom are inherently slightly polar therefore generally are better solvents. The hydrofluorocarbon (HFC) refrigerants, having no chlorine atom, are inferior solvents thus making finding compatible/miscible lubricating oil difficult, for those of high viscosity, even more harder.
In addition to finding lubricating oil that is capable of providing better hydraulic sealing, wear protection as well as superior compatibility/miscibility in order to enhance compressor system efficiency, it is essential that these innovative lubricating oils are to preserved those properties typically possessed by the conventional refrigeration machine lubricating oils such as adequate hydrodynamic lubricating film thickness, high dielectric strength, superior hydrolytic stability, good thermal stability, high flash point, low pour point, and high viscosity index (VI), etc.